The present invention relates to an oxygen delignification method and apparatus in which wood pulp and oxygen are reacted in the presence of caustic soda such that the caustic soda is mixed with the wood pulp in a plurality of mixing stages, the wood pulp is reacted with the oxygen in a plurality of reaction stages located between the mixing stages and filtrate, produced from a washing stage, is mixed with the wood pulp in the mixing stages along with the caustic soda. In another aspect, the present invention relates to a wood pulp mixer having coaxial external, intermediate and internal passageways. The intermediate and internal passageways are provided with perforations sized to retain the wood pulp such that a gas circulated through the external, intermediate and internal passageways mixes with the wood pulp while the wood pulp is driven between the intermediate and internal passageways.
In the production of paper, wood chips are treated with cooking liquor to form wood pulp. In order to produce an unpigmented wood pulp, lignins from the pulp are removed in a process known as oxygen delignification. Subsequent bleaching stages are used to further remove pigments from the wood pulp. Oxygen delignification is carried out by mixing steam with the wood pulp. Thereafter, caustic soda derived from oxidized white liquor is mixed with the wood pulp. The heated wood pulp is then reacted with the oxygen and in the presence of the caustic soda. These foregoing operations allow the lignin to be dissolved from the pulp fiber by a solvent (normally water) in a subsequent washing stage.
After treatment with oxygen, the wood pulp is introduced into the bottom of a treatment tower in which the wood pulp is vertically driven and removed from the top. Passage of the wood pulp through this tower takes approximately one hour. After removal from the tower, the wood pulp, as mentioned above, is washed to produce a filtrate. The filtrate, is often mixed with weak black liquor being discharged from the initial treatment of the wood chips.
The rate of delignification is dependent upon the pH during reaction of the wood pulp and the oxygen. The higher the pH, the greater the degree of delignification. This is not without limit in that a point is reached at which the cellulose is attacked by the caustic soda to cause degradation of the wood pulp. In practice, a charge of wood pulp is mixed with a charge of caustic soda. The wood pulp is then reacted with the oxygen and during such reaction, the caustic soda is being neutralized with acidic reaction by-products to lower the pH during the reaction. Therefore, the rate of delignification decreases during the reaction due to the neutralization of the caustic soda during the reaction. The degree of delignification cannot, however, be increased by supplying a greater initial charge of the caustic soda because of possible pulp degradation and therefore, the delignification of any charge of wood pulp is limited by initial peak pH exposure of the wood pulp to the caustic soda.
As will be discussed the present invention provides an oxygen delignification method in which a greater amount of delignification for a given charge of wood pulp is possible as compared with prior art oxygen delignification methods. Additionally, the present invention provides an apparatus for conducting oxygen delignification that effects a simplification over prior art methodology and apparatus.